1. Field of the Invention
The present invention broadly relates to the field of photolithography; and more particularly, to a method of obtaining a wafer design dimension through a process control system that uses a simple control algorithm and feedback. The feedback utilizes mask design data, mask dimension data and historical wafer feature measurements to center Critical Dimensions (CD) to a wafer feature design dimension. The present invention, utilizing distinct reticle measurements offers a method of controlling the feature size, i.e., Critical Dimension of a line or space, by assuring that the correct exposure conditions are used for each wafer processed.
2. Background of the Invention
In the field of integrated circuits (ICs), photolithography is used to transfer patterns, i.e. images, from a mask containing circuit-design information to thin films on the surface of a substrate, e.g. Si wafer. The pattern transfer is accomplished with a photoresist (e.g., an ultraviolet light-sensitive organic polymer). In a typical image transfer process, a substrate that is coated with a photoresist is illuminated through a mask, i.e., reticle, and the mask pattern is transferred to the photoresist by chemical developers. Hereinafter, the term xe2x80x9creticlexe2x80x9d and the term xe2x80x9cmaskxe2x80x9d may be used interchangeably. Further pattern transfer is accomplished using a chemical etchant.
In current technologies, this masking process usually is repeated multiple times in the fabrication of an integrated circuit.
FIG. 1 illustrates a photolithographic processing (fab) environment comprising a reticle 80 having, for instance, a measured error, i.e., deviation 100 from the design dimension 120, a stepper device 90 with lens 140 through which the exposure condition representing an exposure energy 130 is focused on a wafer 95 coated with a photoresist 150, resulting in a printed wafer at design dimension 160.
It is well known in the field of photolithography that Critical Dimension control is most difficult and challenging in a logic fab where many products are processed simultaneously. With more and more products being introduced into the fab, each with multiple masking layers and unique CD customization, calculations for exposure dose have also increased in complexity. Considering that additional lithography tools and tool types, each with their own calibrations and process variability had to be introduced to track increased volumes and complexity of manufacture, the need for CD control in the fab has transformed into a critical challenge that must be addressed. Given that a significant number of all tool/reticle setups use a particular reticle for the first time, and that a significant number of passes is required to center a product to its target Critical Dimension, there exists a critical need to reduce the errors in the initial production of products using new reticle/tool combinations.
A related art technique for CD control in the fab is described in Adams (U.S. Pat. No. 5,989,764) which is directed to a lithography tool adjustment method through scattered energy measurement. This process however, does not include using reticle size data in a feedback loop to center CD distributions.
Another related art method is described in Hitachi (U.S. Pat. No. 6,225,011) which utilizes a plurality of exposure systems. Again, the reticle size data is not used for Critical Dimension control in this technique.
A further related method disclosed in Kerszykowski (U.S. Pat. No. 5,969,972), involves an automatic machine program generator for use in manufacturing a semiconductor component. While this related art method discloses an optimizer, the optimizer fails to address the need for CD control where a reticle dimension differs from design targets, and where a new reticle has no history with any of the tools that may use the reticle.
Another related art technique described in Marchman (U.S. Pat. No. 5,656,182) utilizes feedback control, however, does not address attainment of the optimum CD. Rather it merely performs stage position control as a function of the latent image produced in the substrate.
While it is well known in the art that an exposure dose bias can be used to compensate for the wafer measurement deviation, and further that reticle factors for previously used reticles can be derived from historical wafer measurements using a feedback exposure control loop, there remains the problem of determining the correct exposure dose bias without the necessity of send-ahead or test wafers for new reticles and products.
FIG. 2 illustrates a feedback system that uses historical dimension data for each reticle and stepper tool. For a mature product, the critical dimension metrology step 210 produces historical data 220 on the mature product which is feedback resulting in feedback calculations 230 which produce the exposure dose setting 240. The mature product does not require a rework step, thus the exposure dose setting 240 is considered to be an optimum dose, calculated using the feedback exposure control loop.
FIG. 3 illustrates the prior art method where there is a new product or new reticle to be used with stepper tool 300. As shown, Critical Dimension Metrology step 310 is performed while there is no historical data from the new tool 305. This requires the first run to use historical data from other tool/reticle combinations 340 which are used in the feedback calculations 360 to produce exposure dose setting 370. Typically, this feedback arrangement does not account for new mask offsets and the resulting product run does not meet design dimension specification 160 of FIG. 1. Any product failing the design dimension specification is reworked 320 allowing later production runs 330 to benefit from production data 350 for the new reticle.
In view of the above mentioned drawbacks with related art techniques, there exists a need for providing CD control which can successfully predict initial exposure doses with new reticle/tool combinations, thereby facilitating reduced cycle times in a high volume, high complexity fab environment. That is, a method is required which is capable of using the measured and design dimensions of a reticle as input to the method, with appropriate feedback parameters, for producing an optimum exposure dose setting.
It is an object of the present invention to provide a photolithographic system and process that enables the reduction of CD errors when producing integrated circuit products using new reticle/tool combinations.
A further object of the present invention is to provide a photolithographic system and process which minimizes the difference between a manufactured CD from an established target CD where there exists a deviation in the actual CD measurement of a reticle versus its design specification.
Another object of the present invention is to provide a method for setting photolithographic exposure doses that achieves a wafer feature size that meets the wafer design specification, i.e. the design dimension +/xe2x88x92 the required tolerance for a given semiconductor product.
A further object of the present invention is to eliminate the necessity for the standard send-ahead process and attendant rework step (320 of FIG. 3) following the required CD metrology step (310 of FIG. 3), thereby reducing process cycle times.
These and other objects and advantages can be obtained in the present invention by utilizing a metrological-feedback method, i.e., a control algorithm, wherein the reticle measurements are used as part of a feedback mechanism for exposure control, including the steps of calculating an initial exposure dose, thereby controlling the CD feature size and eliminating the need for send-ahead wafers. The observed correlation between the actual reticle CD divided by the design reticle CD and reticle factors, i.e. required exposure conditions of a particular reticle relative to the required exposure conditions of similar reticles, of the feedback system is utilized to converge on an optimum exposure dose or exposure condition, i.e. Optimum Dose, in a single pass.
Specifically, for each reticle used in production, reticle factors are calculated and stored in a database. For reticles lacking suitable recent historical wafer data on a given photolithographic tool, the calculated reticle factors are then used to arrive at an optimum exposure condition. However, for a new reticle a correlation is computed between size deviations of similar reticles and their reticle factors. Since the measured dimension and design dimension of the new reticle is known, a derived reticle factor is xe2x80x9cpicked offxe2x80x9d from the historical correlation data. The derived reticle factor is then seeded into a feedback exposure control loop and used to calculate an initial exposure condition for the new reticle.